1. Technical Field
This invention relates to enhancements of public switched telephone network architecture.
2. Description of the Related Art
The Public Switched Telephone Network (PSTN) has a very unique feature that it provides to each and every subscriber a fairly uniform set of capabilities that other forms communication services could not match. For example, every subscriber is identified by an unique telephone number. Without any prior appointment, any subscriber can contact another subscriber at any time by simply dialing the corresponding telephone number from a telephone instrument. This level of flexibility and convenience has made the PSTN the most depended upon form of communication facility in modern life.
The PSTN has evolved, from an initial simple goal of providing voice communication between two individuals over a century ago, to the current state of sophisticated information exchange network among the subscribers. This history of the PSTN being able to successfully keep pace with the evolution of human society is an amazing record.
One significant architectural enhancement to the PSTN was prompted by the expansion of business. When a caller reaches a business nowadays, there are more than one people working there. And, a particular person may be not at the desk but somewhere on the premise. Thus, a localized switching machine privately owned by the business (Private Branch/Business exchange--PBX) is installed at the business location to provide the additional routing capability. Initially, this local switching function was managed by the business' own attendant (one or more full time operators, or as part of a receptionist's duties). With the introduction of voice announcement prompter and Dual Tone Multi-Frequency (DTMF) remote signalling, the Automated PBX (PABX) became available. As far as a caller is concerned, upon hearing the voice prompt from the PABX, the selection of an extension number, commonly referred to as "secondary dialing", is pretty much accepted as the "extra digit(s)" in reaching a person's business telephone. Thus, the PSTN system architecture has been expanded outward to business entities beyond the local end office, traditionally referred to as the Central Office (CO).
Another important aspect of PSTN's evolution is its ability to carry data traffic besides the originally intended voice communication. This was accomplished by utilizing MODEM's (MOdulator & DEModulator) that convert data information from computers into voice band signals for transmission.
Both of these were well accepted in the business environment where calls are greeted by an attendant, either human or mechanized, then directed to the ultimate station instrument based on the caller's selection. And, computers using separate station lines are treated like additional office telephone sets by PABX. Thus, the PSTN, in conjunction with PABX's has been serving voice and data communication needs of the business entities very well.
On the other hand, there have been compromises along the way in many areas. As time goes on, they have built up to begin impeding the PSTN's ability to continue offer its functionalities uniformly to serve the general public towards the future.
As residential subscribers become similarly sophisticated, various types of station instruments, from multiple extension telephone sets, answering devices, facsimile machines, personal computer modems, alarm systems to Home Automation (HA) systems are often used in the same residential setting. Although each of these devices may be used occasionally and the combined traffic can hardly justify an additional telephone line, every device need be constantly connected to the line so that it can perform the designated function when it is called upon. This has worked well on calls originated locally. However, since each station instrument is designed with the assumption that it is the only instrument on the telephone line, this multiple device configuration has resulted in frequent operational confusions on incoming calls and caused irritations to callers.
With more and more home based small business being set up and the vision for a nation wide information superhighway on the horizon, the need for a flexible switched communication network that can serve both voice and data traffic to and from, not only business subscribers but also residential subscribers has become a real issue. If the current PSTN can not satisfy the need, much more expensive new networks have to be deployed. Even if such a technology can be identified, its deployment will take some time to reach the PSTN's current level of "universal connectivity" coverage. This target is much easier said than done, because most of the residential subscribers are very much content with the basic Plain Old Telephone Service (POTS), commonly known as "Dial Tone Service". There will be very little reason for these subscribers to jump onto the new band wagons, no matter how attractive they may be. As a result, the national information superhighway, if it is based on one of the new technologies, will not be accessible by the general public, but restricted to only a selected group of subscribers. Consequently, the population will be divided by whether or not a person has access to the information network (the World Wide Web based on Internet). Such a separation will be detrimental to the unity of the nation in the long run.
A class of telephone line sharing devices, commercially known as FAX switch, has reduced the above problems in some incidences. However, its design goal is to serve telephone lines which are set up primarily for FAX or modem transmissions. Voice call is treated as an exception on its operation priority list. Therefore, when FAX switch is used on primary line or single line service, its behavior is often very offending to a voice caller.
The single line module of Modular Automatic Telephone Switching System (U.S. Pat. No. 5,022,069) provided the line sharing service from voice callers'point of view. With optional FAX and/or modem tone detectors, this unit can also provide automatic routing for data types of calls, which are treated as subordinate to (or operating in the background of) the primary voice call processing.
What was implicit in these last two approaches was these additional routing processes can only be executed after a call is answered by one of these devices. This condition is quite disturbing to callers who are unaware of this extra switching process that is required to reach a desired party. For example, callers from foreign countries may be levied by their telephone companies for a "completed" international call that they did not know how to get through to a person.
This constraint has also limited the PSTN from serving as a switched data network for HA related activities. For example, an utility company would like to access a customer's utility meter, by utilizing the customer's idling telephone line at night for remote reading, without disturbing the residents. Such a setup also has to be able to avoid distracting ordinary voice callers.
A Network Interface Unit (U.S. Pat. No. 5,528,675), disclosed along with a protocol of originating a call to be routed through it, has provided the solution to meet these requirements, without the need of any explicit involvement of the PSTN. Thus, the existing PSTN can provide switched voice and data communications between any two subscribers, without adverse effect to ordinary voice callers.
To achieve these goals without any modifications to the existing PSTN, this latest invention made a conscious performance trade-off. It operates on the principle of blocking the initial two ringing cycles of every call while making the determination of the type of the caller, being an ordinary voice caller or a computer making a data call. This screening process introduces delay in setting up a call, because the called party will not hear the first two rings.
The PABX's (as well as a simpler form of private switching systems, the Key Telephone System--KTS) routinely block the first ringing cycle to minimize false responses to line noises. Also, to fully benefit from the Caller ID service so that the occupants are not disturbed by un-invited callers, incoming call ringing need be blocked until the Caller ID information, which is delivered after the first ringing cycle, is analyzed and matched with the "guest list" in the Caller ID decoder's database. Furthermore, the PSTN's ringing system has a random probability of "missing" one ring or "advance" ringing from the caller's perspective.
Since most calls are made between two parties far apart from each other and without pre-arranged timing, caller usually does not have a definitive prediction of how quickly the called party would hear the ringing and then answer the call anyway. So, the called party's apparent extra delay in response by a couple of ringing cycles can not be sensed by the calling party.
The fact is, ringing signals for any call may be randomly delayed or advanced from the caller's perspective without any adverse effect to the caller. However, explicitly describing to subscribers about the definitive two cycles of ringing delay when U.S. Pat. No. 5,528,675 is practiced, may face some psychological resistance. So, if at all possible, the setup delay of an on-demand data communication system need be cut back to the more commonly accustomed one ringing cycle.
An issue hidden among the above PSTN service enhancements is that the originating subscriber is often required to send command signals to the terminating end to achieve one of these operations. The only reliable signalling method available to the originating subscriber for this purpose is DTMF dialing. Although station instruments with this signalling capability have become very popular in recent years, there are still a huge number of Decadic Pulse (DP) phones in use. Subscribers using DP station sets are increasing get frustrated by their inability to reach stations that can only be accessed through secondary dialing.
It is therefore the object of the present invention to achieve the combined goals of segregating data calls from voice calls, routing voice calls to specific individuals, while reducing the initial ringing cycle delay back to one, and allowing DP instruments to reach specific station on the terminating site. The trade-off in the present approach is that it has to rely upon one of the PSTN capabilities. Although eventually this will be more efficient than the technique disclosed by U.S. Pat. No. 5,528,675, this new system will take some time to begin its usefulness because it depends upon sufficient number of PSTN operating companies to deploy the required enhancement. As it will become clear from the disclosure below, the actual implementation would be simply upgrading the Central Office switching machine's stored programs. There is no hardware facility modification necessary.